This invention deals with the improvement of the cement mortar and concrete, particularly the false set in ground cement and its contraction. The term "cement" as used herein is understood to be the known five types of portland cement and blended hydraulic cement; such as blast-furnace cement, slag cement, fly-ash cement and portland-pozzolan cement. It is the object of this invention to improve the retarder in cement by reducing the amount of gypsum used and substituting from about 0.5 to 5.0 weight percent of stannous sulfate to the cement clinker in the process and manufacture.
Gypsum (CaSO.sub.4 2H.sub.2 O) is commonly used in the amount between 3 to 7% by weight of cement clinkers as a retarder to regulate the hardening rate of cement, but also causes many problems in modern cement manufacturing. During the process of grinding cement clinker, the friction inside the grinding mill generates heat. When it reaches to a certain temperature, the heat will cause the gypsum to dehydrate to form calcium hemihydrate (CaSO.sub.4 1/2 H.sub.2 O) or soluble calcium sulfate (CaSO.sub.4) or both, which upon the addition of water dissolves rapidly and precipitates as gypsum to produce a false set, otherwise known as premature stiffening in a hydrated cement. Sometimes the false set occurs in the product after being aerated or in storage. Since only a small amount of the total mixing water is required for the recrystalization of gypsum, the stiffened mix can be reworked to its original plastic condition with little loss of its workability. In some worst cases, they are not reworkable back to their original workabilities. By reducing the amount of gypsum to a minimum from about 1 to 3 percent and substituting the easily soluble stannous sulfate to regulate the hardening rate, much less dehydrated gypsum product can possibly be formed to cause a false set to occur in the mortar or concrete during hydration of the cement when mixed with water.
The major compounds found in portland cement are tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite. The tricalcium silicate and dicalcium silicate both reacts with water to form hydrated calcium silicate known as tobermorite and calcium hydroxide but the latter reacts at a much lower rate of reaction. Tricalcium aluminate contributes to the hardening rate of the cement, and to cause the hydrates to form hydrated calcium aluminate and calcium hydroxide. In a solution containing calcium sulfate from gypsum which is saturated with calcium hydroxide, tricalcium aluminate will form either a high or a low sulfate form of calcium sulfoaluminate. This change of aluminate to sulfoaluminate retards the hardening rate of the cement. Tetracalcium aluminoferrite which reacts less instantaneously than the tricalcium aluminate, may be expected to form a solid solution of the high sulfate form of sulfoaluminate and sulfoferrite.
In view of the hydration of the major compounds in cement, every compound gives off a similar calcium hydroxide as a final product. Thus a hydrated cement is highly alkaline and saturated with calcium hydroxide. In a calcium hydroxide solution, stannous sulfate precipitates as stannous hydroxide, but in a solution highly saturated with calcium hydroxide, stannous hydroxide tends to form stannous oxide plus water as in the following equations: EQU SnSO.sub.4 + Ca(OH).sub.2 = Sn(OH).sub.2 + CaSO.sub.4 EQU sn(OH).sub.2 = SnO + H.sub.2 O
it is indicated that the use of stannous sulfate, water is formed instead of hydroxide during the hydration, thus reduces the mixing water used for cement mortar and concrete.
The cause of shrinkage in cement has not been clearly determined. There are various theories which fall into three groups: The capilliary tension theory, surfaced adsorption of water and interlayer water in crystals. It has been experienced that during the shrinkage of cement mortar and concrete, moisture is lost. Another experienced phenomenon is that cement consists of the same chemical composition with the same mixture composed of the same kind of sand and gravel, does not give the same shrinkage. Even the difference in size of a clinker, in the same product from the kiln, has a different shrinkage. The explanation for such experience has to trace back to the burning of cement clinker. Aside from the composition of a cement clinker, the process of burning is one of the main factors in the manufacture of a good quality cement. The structure of silicate varies in the degree of burning, thus varies its hydration. There are many factors in the operation of a kiln. The temperature of the burning zone, the rate of the kiln revolution and the retention time of clinker in the burning zone which varies the size of clinker; and the proper clinker cooling are the essential requirements to determine the quality of a good clinker. This invention of using stannous sulfate reduces the use of gypsum, thus decreases the formation of calcium hydroxide and the water requirement for hydration process; therefore less water is lost during the shrinkage process.